There are several thousand active oil and gas wells located around the world, with thousands more to come on-stream in the next ten years. The wells differ in design, size, cost and economic benefit, but have one thing in common—sooner or later they will be decommissioned and abandoned.
The decision to plug and abandon (P&A) a well or field is invariably based on economics. Once production delivers less than the operating expenses, it is time to consider abandonment. In some situations, the decision is made with the knowledge that considerable reserves remain, but the cost to extract these resources is more than the projected income.
There are regulatory requirements associated with the P&A process to ensure that strata, particularly freshwater aquifers, are adequately isolated. The plug's length, cross-section, position and verification tests are typically regulated and depend on the type and location of the well being plugged. Thus, the cost to P&A a well can vary by many millions of dollars depending on location, and whether the well is offshore or onshore. Minimizing costs, without sacrificing well integrity, is critical to operators, who make a significant investment with no financial return in the case of P&A operations.
FIG. 1 displays a simple schematic of a basic plug. A modern requirement for a permanent well barrier is that it must include all annuli, extending to the full cross section of the well and seal both vertically and horizontally. In FIG. 1, a cement plug (104) is sealing vertically inside the casing and sealing both horizontally and vertically in the casing-formation annulus (102) above the casing shoe (103). However, cement is easily contaminated with mud, which results in placement issues and often results in plug failure.
Because cement is susceptible to failure if contaminated by drilling or other fluids, yet long length cement plugs are required, other materials have been investigated for use as plugging material. Resins seal by adhesion, and have resistance to many caustic and corrosive chemicals, excellent mechanical properties, such as high strength and high shear, and flexibility and toughness after setting. However, resin plugs can be more difficult to set successfully because of the relatively complex chemistry and the need for time to cure. Further, resin plugs will fall down the well bore and annulus unless the bore and annulus are plugged. They must be set on a base and the annulus sealed. Shrinkage of the resin can also occur as it cures unless formulated correctly and can lead to micro-annuli and cracks in the sealant and/or lack of bonding of the seal, plug or connection to its surroundings.
Although cement and resin are the most common plug materials, improvements in plug composition and methods are desired to reduce costs. One alternative is to “cast-in-place”a metal plug. A eutectic alloy, such as an expandable bismuth alloy, is heated in place to form a molten liquid metal that easily penetrates small crevices and cracks, and hardens in place forming a tight plug. The specific gravity of the metal being much higher than any fluid, results in effective placement. A heater tool, described in WO2011151271 and WO2014096858, is commercially available from BiSN Oil Tools, and can heat such cast-in place bismuth alloy plugs. The BiSN Wel-lok M2M Bridge Plug™ can run on standard wireline, slick line or coil tubing. It uses a bismuth-based alloy that is melted in situ by a chemical reaction heater that uses materials, such as thermite, to generate heat. However, this tool can only heat a limited volume of alloy at a time, and thus other methods and tools are still needed for P&A.
Thus, there still exists a need to improve plug formation in P&A operations while also decreasing cost and time. Ideally, the new plugs would be safe, create a reliable barrier, be cost effective, and both faster and easier to perform than current methods.